The development of the theory for the many aspects of the magnetic self-focusing of electron beams (see Bennett, "Magnetically Self-Focusing Streams" Phys. Rev. Vol. 45, 1934, pp. 890-897) began in about 1932 and has been continued through the years (see Bennett (with Roberts) "The Pinch Effect in Pulsed Streams at Relativistic Energies," Plasma Physics, Vol. 10, 1968, pp. 381-389, for more references).
The invention of the controlled thermonuclear fusion of deuterium in about 1951 required the containment and heating by compression of ionized deuterium gas and the principal methods used in the succeeding years have depended for the most part upon the various forms of the pinch effect. After the investment by the United States and other nations of a great amount of effort and hundreds of millions of dollars in this work, it became evident to me that ionized gas probably could not be confined long enough to produce a useful amount of thermonuclear energy before the confinement broke up due to an exasperating variety of instabilities that afflicted the containment mechanisms.
At about this time (circa 1964), a new kind of electron beam producing machine was invented by J. C. Martin in England, and developed in the United States, which could produce currents of more than 20,000 amperes at more than 5,000,000 volts in pulses of duration of less than 200 nanoseconds. (See as References Graybill et. al., Machine developed by Ion Physics Corp., Burlington, Mass, J. App. Phy. Letters, Vol. 8, 1966, p. 18, and Pay, Machine developed by Physics International Company, San Leandro, Calif., Technology Week, 1967, p. 10.) I immediately suggest that if an electron beam of 100,000 amperes at 10,000,000 volts could be concentrated to the order of 0.1 millimeter radius and directed into a solid state deuterium-rich material, a very practical and useful method for the production of energy by the thermonuclear fusion of deuterium would result much before any kind of instability could form and interfere. To test this possibility, I initiated experimental work in 1964 and have continued that work ever since. Several U.S. patents have been issued to me for this work; see:
U.S. Pat. No. 3,510,713 May 5, 1970, PA1 U.S. Pat. No. 3,516,906 June 23, 1970, PA1 U.S. Pat. No. 3,526,575 Sept. 1, 1970, PA1 U.S. Pat. No. 3,610,989 Oct. 5, 1971, PA1 U.S. Pat. No. 3,369,849 Feb. 1, 1972, PA1 U.S. Pat. No. 3,864,640 Feb. 4, 1975.
In the course of this work, I have concentrated electron beams to less than one millimeter diameter and projected them through thin metal foils into room air at atmospheric pressure where each such beam is quite visible and is observed to hold its diameter of less than one millimeter while traveling more than fifty millimeters to a three millimeter thick aluminum target in which it blasts a hole more than six millimeters in diameter.
In experimental work of this kind, the usual practice has been to use a cylindrically symmetrical diode in which the beam is to be formed, which consists of a conducting tube envelope with a cathode support, a cathode, and the beam all along the axis of symmetry. The distance from the cathode support and cathode to the surrounding envelope which is at anode potential must be great enough to avoid excessive destructive transverse field emission and the self-magnetic field of the current must confine the discharge to the beam at and near the axis.
In this work, by the time the beam has concentrated to a little less than a millimeter in diameter, the total energy stored in the self-magnetic field surrounding the beam is approaching the total energy stored in the machine which is driving the discharge and producing the beam so that the beam cannot spontaneously concentrate itself further in this kind of experimental arrangement. Changes in this system, as set forth below, had to be introduced to further reduce the radius of the beam to the order of one-tenth millimeter (100 microns) as required to produce the many very important applications.